PSBTv2 (BIP-370) Support

[notTanveer]

This PR improves PSBT (Partially Signed Bitcoin Transaction) v2 support, especially strict validation and compatibility with BIP 370 test vectors. It also adds a comprehensive set of tests for PSBTv2 edge cases and locktime determination.

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Details

1. PSBTv2 Compliance and Parsing Improvements

• Updated method signatures in src/embit/liquid/pset.py to allow passing the version parameter down to read_value and its parent implementations. This ensures correct handling of versioned PSBT fields.
• Adjusted calls to super().read_value to pass along the version argument, improving extensibility and correctness for PSBTv2.

2. PSBT View Updates

• Modified src/embit/psbtview.py:
  • The input() and output() methods now pass the version parameter when reading from the stream, allowing PSBT-in and out scopes to parse versioned fields correctly.

3. Comprehensive PSBTv2 Test Vectors

• Added a new test file: tests/tests/test_psbtV2.py containing:
  • Test vectors based on BIP 370 PSBTv2.
  • Tests for invalid PSBTv0/PSBTv2 edge cases (e.g., wrong/missing fields, invalid locktimes).
  • Tests for valid PSBTv2 minimal and full-featured transactions.
  • Tests for locktime determination logic in PSBTv2, including edge cases with multiple locktimes (height and time).

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Motivation

• Ensures full BIP 370 compliance, increasing interoperability and correctness for PSBTv2 parsing and validation.
• Catches subtle bugs and regressions through comprehensive test coverage.
• Lays the groundwork for future PSBTv2 features and broader Elements/Liquid support.

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Testing

• Run pytest tests/tests/test_psbtV2.py to verify all BIP 370 test vectors and locktime cases.
• Built from the referenced PR and generated various PSBTs, then imported them into embit to confirm correct parsing — everything worked as expected.
• Would love for others to build Bitcoin Core + CLI from PR #21283, try it out, and share your observations here.

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References

• BIP 370: PSBTv2
• Bitcoin Core PR #21283